The present invention relates to toner powder for use in electrophotography, and, more specifically, to a heat-fusible toner composition used for the development of electrostatic latent images such as those formed on an insulating or photoconductive insulating surface in the electrophotographic imaging arts.
In electrophotographic printing, generally a uniform electrostatic charge is applied to a photoconductive insulating layer and the resulting charged surface selectively exposed to electromagnetic radiation so as to dissipate the charge in those areas exposed to the radiation thereby producing an electrostatic latent image. The resulting latent image is subsequently developed by depositing a finely divided electroscopic developer material, referred to as toner, on the electrostatically formed image. Generally speaking, the charged toner particles will have a charge opposite to the residual electrostatic charge image so that the toner particles adhere to the charged areas to form a visible image. This image may be fixed in situ on the support or transferred to a secondary support surface and the transferred image permanently affixed to the secondary support surface.
Two component developer mixtures are conventionally used to develop the images comprising a pigmented resinous toner powder and a carrier component wherein the carrier component is substantially larger in size than its toner complement. The toner particles, which are generally made up of a fine pigmented resinous material, are charged triboelectrically by rubbing against the carrier particles causing them to adhere electrostatically thereto. The composition of the developer mix is chosen such that the toner particles will acquire an electrostatic charge of a polarity opposite to that of the electrostatic image to be developed. As a result, when the developer is brought into contact with the electrostatic latent image, the toner particles are attracted from the carrier particles and selectively deposited onto the electrostatic image by the electrostatic charge of the image. The powder or toner image that is obtained is either fixed in situ on the surface of the image-bearing substrate or the powder image selectively transferred to a receiving surface to which it is then fixed. The fixing process can reflect any one of several approaches such as pressure fixing, vapor fixing or heat fusing, depending upon the specifics of the particular system. In another form of development utilizing a developer mix comprising a carrier and toner component, a developer composition containing toner and magnetic carrier particles is transported by a magnet. The resulting magnetic field causes alignment of the magnetic carrier into a brush-like configuration. This magnetic brush is engaged with the electrostatic image-bearing surface, and the toner particles supported on the brush-like configuration, are drawn from the "brush" to the latent image by electrostatic attraction. Thus, a developer mixture may be provided comprising a toner material and a carrier material which consists of particles which are magnetically attractable. Such a configuration is generally referred to as a magnetic brush development system.
Although the above development systems have been found useful in electrophotography, they are not without their disadvantages. For example, in the use of a developer mix comprising both the toner and carrier components, in combination with a heat-fusible image fixing system, mix fatique is encountered which generally results in poor copy images and the masters produced therefrom have relatively short periods of usage time. The carrier component slowly is reduced in size and becomes increasingly coated with toner, producing a resulting change in the triboelectric charge relationship, thus leading to a lower quality image. Furthermore, in the reproduction of high-contrast copies utilizing the powder-carrier developer mix wherein the mutual electrification of the components is governed by the distance between their relative positions in the triboelectric series, when otherwise compatible electroscopic powder and carrier materials are removed from each other in the triboelectric series by too great a distance, the resulting images are very faint because the attractive forces between the carrier and toner particles compete with the attractive forces between the electrostatic latent image and the toner particles. It is, therefore, readily apparent that many materials which otherwise have suitable properties for employment as carrier particles are unsuitable because they possess unsatisfactory triboelectric properties. In addition, uniform triboelectric surface characteristics of many carrier surfaces are difficult to achieve with mass production techniques.
Heretofore, a single component toner material has been provided for use in combination with pressure-fusing systems which eliminates the need for the presence of the carrier component, with a certain degree of success, since the toner, as a result of its formulation, including a magnetic component, serves as its own carrier and, thus, is useful in the development of electrostatic latent images in electrophotography. However, due to the preferred use of heat-fusing techniques in fixing the resulting developed image to produce a more reliable and permanent image, it is preferred to utilize a single component toner material which is compatible with a heat fusing system so as to enhance the results of the specific electrophotographic imaging system.